Recently, it has been highly required that a large volume of data can be processed at a higher speed along with the development of digital technologies. In addition, it has been also required that semiconductor devices used in electronic equipment be further densely integrated and show higher performance. Accordingly, there has been widely studied technologies that use, to provide a semiconductor storing device (DRAM: a dynamic random-access memory) with higher integration, ferroelectric materials or high dielectric materials in place of conventionally used silicon oxides or silicon nitrides as a material of an insulating film of capacitor in the DRAM.
A ferroelectric random access memory (FeRAM) is a non-volatile semiconductor memory device using a ferroelectric for an insulating film (a capacity insulating film) of a capacitor and stores data by utilizing a hysteresis characteristic of the ferroelectric. The ferroelectric has a characteristic that polarization is caused when a voltage is applied and that spontaneous polarization is maintained even after the voltage is stopped to be applied. In addition, when the polarity of the applying voltage is reversed, the polarity of the spontaneous polarization is also reversed. Accordingly, it is possible to store data by associating one polarity with “1” and the other polarity with “0”. The stored data can be read out by detecting a polarity of the spontaneous polarization.
The ferroelectric film constituting the capacitor of FeRAM is made of lead zirconate titanate (PZT), PLZT in which La is doped into PZT, a PZT-based material in which a small amount of Ca, Sr, or Si is doped, Bi layered structure compounds, such as SrBi2Ta2O9(SBT, Y1) and SrBi2(Ta, Nb)2O9(SBTN, YZ), and is formed by a sol-gel method, a sputtering method, a metal organic chemical vapor deposition (MOCVD) method, or the like.
In general, an amorphous or microcrystalline ferroelectric film is formed on a lower electrode by using these film forming methods, and thereafter a crystal structure thereof is changed by thermal processing to have a perovskite structure or bismuth layered structure. As a material for an electrode of a capacitor, it is required to use a material which is less likely to be oxidized or which can maintain conductivity even after being oxidized. In general, platinum metals such as platinum (Pt), iridium (Ir), iridium oxide and the like, or oxidants thereof are widely used. In addition, aluminum (Al) is a generally used material for a wiring as in the general semiconductor device.
Similar to other semiconductor devices, FeRAM is also required to be further densely integrated and to show higher performance, so that it is needed to reduce an area of a cell in the feature. To reduce the area of the cell, it has been known that it is effective to use a stack structure in place of a conventional planar structure. Here, the stack structure means a structure that a capacitor is formed directly on a plug (a contact plug) formed on a drain of a transistor constituting a memory cell. In a conventional FeRAM with a stack structure, a capacitor is formed directly on a W (tungsten) plug by laminating a barrier metal, a lower electrode, a ferroelectric film, and an upper electrode in this order. The barrier metal has a function to prevent the W plug from oxidation. Since a material having the effect of the barrier metal together with the effect of the lower electrode is often selected, the barrier metal and the lower electrode cannot be clearly separated. However, the barrier metal and the lower electrode are generally formed by combination of at least two films, which are selected from a TiN film, a TiAlN film, an Ir film, an IrO2 film, and an SRO(SrRuO3) film.
In Japanese Patent Laid-open No. 2000-31421, for the purpose of preventing excessive necking of a storage node, it is disclosed that a recessed W plug filling a part of a contact hole is formed, and thereafter a conductive film (for example, a Cu film) and an insulating film (for example, SiON film) are sequentially formed on an entire surface thereof, and then the conductive film and the insulating film are polished by chemical mechanical polishing (CMP) to form a contact plug with a structure in which an insulating material is buried in the center portion thereof.
In addition, In Japanese Patent Laid-open No. Hei 10-242423, there is disclosed a semiconductor device in which a second plug formed by filling a contact hole in a second interlayer insulating film with W is laminated on a first plug formed by filling a contact hole in a first interlayer insulating film with polysilicon, and a capacitor is formed on this second plug.
Furthermore, In Japanese Patent Laid-open No. 2003-68993, it is proposed that a W plug is formed after a hydrogen permeation prohibiting film made of TiAlN, TiN, TaN or the like is formed on an inner surface of a contact hole to avoid deteriorating characteristics of a capacity insulating film at the time when processing is carried out in a high concentration hydrogen atmosphere.